The invitation relates to a method and to a device for qualitative determination of the cavitation energy in a container, in the liquid volume of which cavitation is initiated by a suitable ultrasonic source. The smallest cavities microscopically are hereby formed constantly and visibly rise coagulating or also implode. As a result of this production of gas or vapour bubbles, cavitation leads to a measurable expansion of the liquid. In order to determine the cavitation energy, a hermetically sealed measuring chamber with a sound-permeable window region is disposed in the container. During operation of the ultrasonic source, the increase in volume of the measuring liquid, initiated by cavitation, can be determined in the measuring chamber with a short-term measurement. The relative increase in volume of the measuring liquid thereby corresponds in a good approximation to the cavitation energy introduced proportionally into the measuring chamber for the container. The cavitation energy crucially determines the ultrasonic effect and hence also for example the effective cleaning power of an ultrasonic bath. Via the relative increase in volume, for example the ultrasonic cleaning power produced by the container of an ultrasonic bath can be tested readily.
In ultrasonic cleaning and surface technology and also in medical fields and in industry, ultrasonic tubs or also other containers have been used for decades, which are equipped with the most varied of ultrasonic sources. In the case of typical ultrasonic baths these are ultrasonic converters fitted on the base or on the sides of the ultrasonic tub; however there is also a large number of other devices, such as e.g. immersible transducers, oscillating plates, tubular or bar-shaped ultrasonic converters, which are introduced into liquid containers in order there to initiate specifically the preferred ultrasonic cavitation. In the case of converters, preferably low-frequency ultrasonic systems are thereby used with operating frequencies between 18 kHz and 500 kHz since the cavitation initiated by them is greatest at low ultrasonic frequencies. The cleaning effect or the dirt-detaching effect of the cavitation on the surface of parts is then at its greatest. However, also different dispersing, de- and emulsifying and also sonochemical effects are greatest at a low operating frequency.
The central problem for many users of ultrasound is the requirement to be able to test their cleaning or cavitation power. Since direct measurability of the cavitation or cavitation energy which is introduced into the liquid by ultrasound is very difficult, alternative methods have been developed.
A current and very inexpensive possibility is represented by the so-called foil test according to IEC/TR 60886. A very thin aluminium foil is stretched hereby on a wire frame and retained diagonally in the ultrasonic bath for a defined, always constant time span. In the presence of cavitation, the foil is visibly attacked and planar perforations and/or holes are formed. When implemented at different points in time and under the same conditions, foil patterns of this type offer a basis for assessment of a constant or decreasing cleaning power in the ultrasonic bath. The reproducibility of the measuring conditions which is not always simple and also the fact that the foil itself changes the sound field in the ultrasonic bath are disadvantageous.
Further methods reside in detecting locally, by means of a small sound pressure sensor, the noise change pressure amplitude at a specific location in the ultrasonic bath and recalculating this value into an energy-equivalent value and displaying it.
The sound pressure sensor can thereby contain a single piezo element in bar form or has a sound pressure-sensitive membrane surface with a plurality of sensors. “Ultrasonic energy meter” or “cavitation meter” of this type are marketed for example by the company ppb/USA and are described also for example in the patent specifications U.S. Pat. No. 313,565 and U.S. Pat. No. 6,691,578.
A typical sensor arrangement with a plurality of piezo sensors on a quartz disc is described in U.S. Pat. No. 6,450,184. The described measuring apparatus for an ultrasonic bath is intended to serve for picking up the “cavitation profile” close to the surface of parts to be cleaned, for example wafer discs. The local distribution of the “cavitation energy” is intended to be determined from the summated electrical signals.
Common disadvantages of these technologies are the dependence of the measuring values upon the respective measuring location and also the sound field change caused by the measuring device itself. In the case of measurements by hand with single sensors, there is also an individual error component due to changes in position. In addition, the measured sound pressure offers no information about the cavitation since the context is non-linear to a high degree.
In order to test the cleaning function of ultrasonic baths also small test tubes are used. The test tubes are thereby placed in the ultrasonic bath at various positions in a cleaning basket. In the presence of cavitation, the test tubes show a colour change from green to yellow after a specific time. This is intended to be based on a cavitation-dependent pH value change in the solution in the tube which also still contains the smallest glass balls as cavitation-initiating nuclei. It should be borne in mind that the colour change is dependent upon the local ultrasonic intensity, upon the bath temperature and upon the ultrasonic frequency. If no colour change occurs, this does not automatically mean that no cavitation is taking place at the occupied position. Allegedly, it acts more at the outer surface of the tube but does not suffice to initiate the colour change. For spontaneous quick testing as to whether an ultrasonic device is still cleaning at all, test tubes can possibly be used. For qualitative assessment of the cleaning power of an ultrasonic bath, they are not suitable.
For more precise tests for the cleaning power of ultrasonic baths, occasionally measuring devices with broadband hydrophones are also used. In DE 102006026525, such a method is described for example. The cavitation-caused noise signals are thereby picked up by the hydrophone at various points in the ultrasonic bath, averaged and evaluated spectrally. The method and the required measuring and evaluation technology is very complex and not suitable for a routine testing of ultrasonic baths.
GB 2 147 104 A shows a measuring chamber for determining the energy generated from an ultrasonic source via the increase in volume of a measuring liquid contained in the measuring chamber, the measuring chamber having a container for receiving the measuring liquid with a sound-permeable window region and a sensor for measuring the increase in volume of the measuring liquid. In the preferred embodiments, an increase in volume of the bubble-free liquid which is then measured is achieved by heating. As an alternative, a cavitation measurement is mentioned.
SU 1 196 696 A1 discloses a measurement via a capillary tube.
Additional prior art is shown in DE 44 10 032 A1, U.S. Pat. No. 3,443,797 A and also US 2003/087748 A1.